This invention relates to a method of compensation in thermal recording. More particularly, it relates to a method of performing compensation in thermal recording with thermal recording apparatus by which the unevenness of a recorded image on a film is measured optically and compensated on the basis of the result of measurement.
Image recording apparatus that perform image recording on recording media with a thermal head are used extensively. In this type of image recording apparatus, a thermal recording material as a recording medium is pressed against a line thermal head having a multiple of heat-generating elements arranged in a 1D direction and as they are individually controlled in accordance with image data, the thermal recording material is transported in a direction perpendicular to the 1D direction, thereby recording the desired 2D gradation image.
The formation of various gradation images is depicted in FIG. 4A. An image with a gradation (D) of 1 is formed by heating the heat-generating elements for t seconds. An image with D=2 is formed by heating for 2 t seconds. Similarly, images with D=3, 4 and 5 are formed by heating for 3 t, 4 t and 5 t seconds.
As a result, pixels are formed on the thermal recording material and the area of color formation is gradation-dependent within the range of one pixel width in the direction of transport (see FIG. 4B), whereby a gradation image is recorded. While recording is performed by pulse-width modulation in the case under consideration, it should be noted that gradation images can also be recorded by pulse-number modulation in essentially the same manner.
If image is recorded using image data for the same specified recording density (gradation), so-called shading occurs from the thermal printer as unevenness in the recording density (a problem generally characterized in that image density is the highest in the center area of the thermal head in the direction in which the glaze extends but gradually decreases toward either end). So-called shading compensation is effected in order to correct this unevenness in density that occurs in the above-described type of image recording.
To perform shading compensation, image is recorded using image data for the same specified recording density and the density of the recorded image is measured optically and on the basis of the measured recording density, shading compensation data are preliminarily computed to enable subsequent compensation of the image data such that the actually recorded image will have a uniform density, and the image data for the recorded image is compensated using the computed shading compensation data.
Since the problem of shading in the thermal recording apparatus results from the thermal head, the site of occurrence of uneven densities in the recorded image does not change. On the other hand, the intensity of unevenness varies with many factors including the recording density of the image data, the temperature of the thermal head and the speed at which image recording is done (the transport speed of the heat-sensitive material relative to the thermal head) and it has been difficult to compensate shading with high precision.
This problem was previously addressed by the assignee and a solution has been proposed in Japanese Patent Application No. 8-42969 xe2x80x9cThermal Recording Apparatusxe2x80x9d (see JP 9-234899 A). Functionally, the proposed technology uses two essential portions, one being a correcting data storage portion which holds image data shading compensation data and weighting functions for weighting the correction coefficients for shading compensation, and the other being an image processing portion which weights the shading compensation data on the basis of the weighting functions, computes the correction coefficients for shading compensation and performs shading compensation on the image data.
As it turned out, however, this method of shading compensation based on optical measurements involves a new problem. That is, if the recorded image has uneven densities at high frequencies, they cannot be completely followed by the measuring optics and only xe2x80x9cdullxe2x80x9d results occur.
If the result of measurement is xe2x80x9cdullxe2x80x9d, it is clear that no further satisfactory result can be obtained by performing shading compensation on the thermal recording apparatus using the compensation data constructed on the basis of such xe2x80x9cdullxe2x80x9d result.
The present invention has been accomplished under these circumstances and its principal object is to improve the method of compensation in thermal recording with thermal recording apparatus of a type that performs optical measurement of the unevenness in the density of a recorded image on a film and which corrects the unevenness of image density on the basis of the result of the measurement. More particularly, the invention provides an improved method of compensation in thermal recording which is adapted to assure satisfactory compensation for uneven densities that occur at high frequencies in the thermal recording apparatus.
In order to attain the object described above, the present invention provides a method of compensation in thermal recording comprising the steps of: performing photoelectric reading of a recorded image on a thermal recording material to construct unevenness data; and using the unevenness data to perform unevenness compensation, wherein the unevenness data constructed by the photoelectric reading is used in the unevenness compensation after the unevenness data is subjected to filtering for frequency enhancement.
Preferably, the filtering for the frequency enhancement of the unevenness data is such that a low-frequency component of the unevenness data is left as it is but a high-frequency component of the unevenness data is enhanced.
Preferably, the filtering for the frequency enhancement of the unevenness data is such that low-frequency component of the unevenness data is left as it is but high-frequency component of the unevenness data is enhanced and linear interpolation in a degree of the frequency enhancement in accordance with a frequency is effected between the low-frequency component and the high-frequency component.
Preferably, the filtering for the frequency enhancement of the unevenness data is performed by mathematical operations on digital data.
Preferably, the unevenness data is constructed by performing the photoelectric reading of the recorded image in which the thermal recording is performed on the thermal recording material using image data for an identical specified recording density.
Preferably, the unevenness compensation is shading compensation.
In order to attain the object described above, the present invention provides a method of compensation in thermal recording comprising the steps of: performing the thermal recording on a thermal recording material using image data representing an image having a uniform density; performing photoelectric reading of a recorded image on the thermal recording material to construct unevenness data; subjecting the unevenness data to filtering for frequency enhancement; and using the unevenness data subjected to the filtering to perform unevenness compensation of a thermal recording image.